A-Level Physics: Structure of the Atom and Fundamental Particles

Introduction

• Overview of nuclear physics history.
• Democritus: Proposed that matter is composed of tiny atoms differing in size, shape, and motion.
• JJ Thompson: Discovered internal structure of atoms; proposed the Plum Pudding model.

Discoveries and Models

• Alpha Particles: Used to probe atomic structure, disproving the Plum Pudding model.
• Ernest Rutherford's Experiment:
  • Directed alpha particles at gold foil.
  • Observations:
    • Most particles passed through: atom mostly empty space.
    • Some deflected: near nucleus interaction.
    • Few reflected: collided with dense, positive nucleus.
  • Significance: Led to Solar System model where mass and charge are concentrated in dense nucleus.

Atomic Scale and Particles

• Atomic Scale: Proton radius ~10^-10; use of unit 10^15 for convenience.
• Protons:
  • Dense particles, ~10^14 times denser than ordinary objects.
• Nucleons:
  • Particles inside the nucleus (protons & neutrons).
  • Nucleon number = number of protons + neutrons.
• Unified Atomic Mass Unit: 1/12 the mass of carbon-12; avoids tiny values.

Atomic Notation and Isotopes

• Nucleon & Nucleide Notation:
  • Specific type of atom defined by protons & neutrons.
• Isotopes:
  • Same protons, different neutrons; same chemical but different nuclear properties.
  • Examples: Hydrogen, Carbon, Chlorine, Uranium, Neon.

Ions and Nuclear Force

• Ions:
  • Positive ion: more protons than electrons.
  • Negative ion: more electrons than protons.
• Nuclear Force:
  • Binds protons & neutrons despite electrostatic repulsion.
  • Acts over short range; instability in large nuclei.

Stability and Radiation

• Nuclear Stability:
  • Stable nuclei have balanced proton/neutron ratio.
  • Over 83 protons: inherently unstable.
• Radiation Discovery:
  • Henri Becquerel: Discovered radiation; uranium emitted energy affecting plates.
  • Alpha, beta, gamma rays emitted by unstable nuclei.

Types of Radiation

• Alpha Particles:
  • Heaviest; two protons & two neutrons.
  • Short range, high ionization.
• Beta Particles:
  • Smaller; one electron; high penetration.
  • Beta-plus: positron (positively charged electron).
• Gamma Rays:
  • Electromagnetic radiation; high penetration, low ionization.

Radiation Characteristics

• Alpha: Slow, heavy, short range.
• Beta: Moderate speed, penetrates more than alpha.
• Gamma: Fastest, most penetrating.

Energy and Radiation

• Energy Units: Electron volt used for small values.
• Energy Distribution:
  • Alpha: Consistent energy.
  • Beta: Varying energy due to neutrinos.

Subatomic Particles

• Discoveries:
  • Cosmic ray interactions and particle accelerators reveal new particles: muons, pions, quarks, gluons, bosons.
• Families of Particles:
  • Leptons: Do not experience strong nuclear force (e.g., electrons).
  • Hadrons: Made of quarks, include protons, neutrons, mesons.

Quarks

• Six types: up, down, charm, strange, top, bottom.
• Quarks combine to form hadrons.
• Protons: 2 up quarks + 1 down quark. Neutrons: 1 up quark + 2 down quarks.

Conclusion

• Understanding subatomic particles and radiation enhances comprehension of nuclear physics.
• Study of atoms, their components, and forces critical for advanced physics.